When growth hormone binds to its receptor, which belongs to the cytokine receptor superfamily, activates the Janus kinase Jak2. Jak2 provides tyrosine-kinase activity and initiates an activation of several
key intracellular proteins (for example, mitogen-activated protein (MAP) kinases) that eventually
execute the biological actions induced by growth hormone, including the expression of particular genes.
In contrast to receptors that themselves have tyrosine kinase activity, the signaling pathways leading to
MAP kinase activation triggered by growth hormone are poorly understood, but appear to be
mediated by the proteins Grb2 and Shc. Growth hormone stimulates tyrosine
phosphorylation of the receptor for epidermal growth factor (Egf-r) and its association with Grb2;
at the same time, Egf-r stimulates MAP kinase activity in liver, an important target tissue of growth hormone.
Expression of Egf-r and its mutants reveals that growth-hormone-induced activation of MAP kinase
and expression of the transcription factor c-fos requires phosphorylation of tyrosines on Egf-r, but not
its own intrinsic tyrosine-kinase activity. The tyrosine at residue 1,068 of the Egf-r is proposed
to be one of the principal phosphorylation sites
and a Grb2-binding site stimulated by growth hormone
via Jak2. These results indicate that the role of Egf-r in signaling by growth hormone is to be
phosphorylated by Jak2, thereby providing docking sites for Grb2 and activating MAP kinases and gene
expression, independent of the intrinsic tyrosine kinase activity of Egf-r. This may represent a novel
cross-talk pathway between the cytokine receptor superfamily and growth factor receptor (Yamauchi, 1997).

The Janus family of protein tyrosine kinases (JAKs) regulates cellular processes involved in cell growth, differentiation and transformation
through their association with cytokine receptors. However, compared with other kinases, little is known about cellular regulators of the JAKs.
A JAK-binding protein (JAB) has been identified that inhibits JAK signaling in cells.
JAB specifically binds to the tyrosine residue (Y1007) in the activation loop of JAK2, whose phosphorylation is required for activation of kinase
activity. Binding to the phosphorylated activation loop requires the JAB SH2 domain and an additional N-terminal 12 amino acids (extended
SH2 subdomain) containing two residues (Ile68 and Leu75) that are conserved in JAB-related proteins. An additional N-terminal 12-amino-acid
region (kinase inhibitory region) of JAB also contributes to high-affinity binding to the JAK2 tyrosine kinase domain and is required for
inhibition of JAK2 signaling and kinase activity. These studies define a novel type of regulation of tyrosine kinases and might provide a basis for
the design of specific tyrosine kinase inhibitors (Yasukawa, 1999).

A mechanism by which members of the ciliary neurotrophic factor (CNTF)-leukemia inhibitory factor
cytokine family regulate gliogenesis in the developing mammalian central nervous system has been
characterized. Activation of the CNTF receptor promotes differentiation of cerebral cortical precursor
cells into astrocytes and inhibits differentiation of cortical precursors along a neuronal lineage.
Although CNTF stimulates both the Janus kinase-signal transducer and activator of transcription
(JAK-STAT) and Ras-mitogen-activated protein kinase signaling pathways in cortical precursor cells,
the JAK-STAT signaling pathway selectively enhances differentiation of these precursors along a glial
lineage. These findings suggest that cytokine activation of the JAK-STAT signaling pathway may be a
mechanism by which cell fate is controlled during mammalian development (Bonni, 1997).

Jak2-deficient mice demonstrate an embryonic lethality due to the absence of definitive erythropoiesis. Fetal liver myeloid progenitors, although known to be present based on the expression of lineage specific markers, fail to respond to erythropoietin, thrombopoietin, interleukin-3 (IL-3), or granulocyte/macrophage colony-stimulating factor. In contrast, the response to granulocyte specific colony-stimulating factor is unaffected. Jak2-deficient fibroblasts fail to respond to interferon gamma (IFNgamma), although the responses to IFNalpha/beta and IL-6 are unaffected. Reconstitution experiments demonstrate that Jak2 is not required for the generation of lymphoid progenitors, their amplification, or functional differentiation. Therefore, Jak2 plays a critical, nonredundant role in the function of a specific group of cytokines receptors (Parganas, 1998).

Janus kinases (Jaks) play an important role in signal transduction via cytokine and growth factor receptors. To date, it is known that Jak2 is associated with receptors binding to the following factors: erythropoietin, growth hormone and thrombopoietin, as well as common beta chain-containing receptors known to bind granulocyte/macrophage colony-stimulating factor (GM-CSF), interleukin-3, and IL-5. Other receptors associated with Jak2 are cardiotrophin-receptor, prolactin-receptor, granulocyte CSF-R, and cytokine receptors containing gp130 in their receptor chain complex. A targeted inactivation of Jak2 was performed. Jak2-/- embryos are anemic and die around day 12.5 postcoitum. Primitive erythrocytes are found, but definitive erythropoiesis is absent. Compared to erythropoietin receptor-deficient mice, the phenotype of Jak2 deficiency is more severe. Fetal liver BFU-E and CFU-E colonies are completely absent. However, multilineage hematopoietic stem cells [CD34low, c-kit(pos)] can be found, and B lymphopoiesis appears intact. In contrast to IFNalpha stimulation, Jak2-/- cells do not respond to IFNgamma. Jak2-/- embryonic stem cells are competent for LIF signaling. The data provided demonstrate that Jak2 has pivotal functions for signal transduction of a set of cytokine receptors required in definitive erythropoiesis (Neubauer, 1998).

Mice lacking the ubiquitously expressed Janus kinase, Jak1, are runted at birth, fail to nurse, and die perinatally. Although Jak1-/- cells are responsive to many cytokines, they fail to manifest biologic responses to cytokines that bind to three distinct families of cytokine receptors. These include all class II cytokine receptors, cytokine receptors that utilize the gamma(c) subunit for signaling, and the family of cytokine receptors that depends on the gp130 subunit for signaling. These results thus demonstrate that Jak1 plays an essential and nonredundant role in promoting biologic responses induced by a select subset of cytokine receptors, including those in which Jak utilization was thought to be nonspecific (Rodig, 1998).

Leptin exerts its weight-reducing effects by binding to its receptor and activating signal transduction in hypothalamic neurons and
other cell types. To identify the components of the leptin signal transduction pathway, an approach was developed in which
bacterially expressed phosphorylated fragments of Ob receptor b (Ob-Rb) were used as affinity agents. Leptin binding to the
Ob-Rb form of the leptin receptor leads to tyrosyl phosphorylation of the cytoplasmic domain of its receptor. Two of the three
cytoplasmic tyrosines of Ob-Rb, at positions 985 and 1138, are phosphorylated after leptin treatment. Affinity chromatography
using a tyrosine-phosphorylated fragment spanning Tyr 985 of Ob-Rb was used to identify proteins that bind to this site. The SH2 domain containing protein tyrosine
phosphatase 2 (SHP-2) was isolated from bovine and mouse hypothalamus by using this method. After cotransfection into 293T cells of Ob-Rb, Janus kinase 2 (JAK2), and
SHP-2, leptin treatment results in direct binding of SHP-2 to the phosphorylated Tyr 985. The bound SHP-2 is itself tyrosine phosphorylated after leptin
treatment. SHP-2 is not phosphorylated after leptin treatment when a Y to F 985 receptor mutant is cotransfected. In the absence of SHP-2
phosphorylation, the level of JAK2 phosphorylation is increased. Tyrosyl phosphorylation of the leptin receptor and signal transducer and activator of transcription
3 (STAT3) are not affected by phosphorylation of SHP-2. These data suggest that activation of SHP-2 by the leptin receptor results in a decreased phosphorylation
of JAK2 and may act to attenuate leptin signal transduction. The data also suggest that the dephosphorylation of JAK2 is a direct action of SHP-2. Thus, a point mutation that ablates SHP-2 phosphatase activity also ablates
its effects on the state of JAK2 phosphorylation. Although SHP-2 does have intrinsic phosphatase activity, it also could lead to dephosphorylation of JAK2 indirectly
by functioning as an adapter protein. For example, binding of SHP-2 to the activated platelet-derived growth factor receptor leads to its own phosphorylation at
position Tyr 584, which in turn leads to binding of Grb2. Grb2 then activates ras and the mitogen-activated protein kinase signaling pathway. Previous studies have
shown that leptin can activate mitogen-activated protein kinase. Indeed the available data are consistent with the possibility that SHP-2 could both decrease
JAK2 phosphorylation and stimulate signaling via the mitogen-activated protein kinase or other pathways.
The method used in this report can in principle be used to isolate additional components of the leptin, or
other, signal transduction pathway (Li, 1999).

What is the effect of the v-abl oncogene (Drosophila homolog: see enabled) of the Abelson murine leukemia virus (A-MuLV) on the Jak-STAT pathway of cytokine signal
transduction? In murine pre-B lymphocytes transformed with A-MuLV, the Janus kinases (Jaks) Jak1 and Jak3
exhibit constitutive tyrosine kinase activity, and the STAT proteins (signal transducers and activators of transcription) normally
activated by interleukin-4 and interleukin-7 are tyrosine-phosphorylated in the absence of these cytokines. Coimmunoprecipitation
experiments reveal that in these cells v-Abl is physically associated with Jak1 and Jak3. Inactivation of v-Abl tyrosine kinase in a
pre-B cell line transformed with a temperature-sensitive mutant of v-abl results in abrogation of constitutive Jak-STAT signaling. A
direct link may exist between transformation by v-abl and cytokine signal transduction (Danial, 1995).

In Abelson murine leukemia virus (A-MuLV)-transformed cells, members of the Janus kinase (Jak) family of non-receptor tyrosine kinases and the signal transducers and activators of transcription (STAT) family of signaling proteins are constitutively activated. In these cells, the v-Abl oncoprotein and the Jak proteins physically associate. To define the molecular mechanism of constitutive Jak-STAT signaling in these cells, the functional significance of the v-Abl-Jak association was examined. Mapping the Jak1 interaction domain in v-Abl demonstrates that amino acids 858 to 1080 within the carboxyl-terminal region of v-Abl bind Jak1 through a direct interaction. A mutant of v-Abl lacking this region exhibits a significant defect in Jak1 binding in vivo, fails to activate Jak1 and STAT proteins, and does not support either the proliferation or the survival of BAF/3 cells in the absence of cytokine. Cells expressing this v-Abl mutant show extended latency and decreased frequency in generating tumors in nude mice. In addition, inducible expression of a kinase-inactive mutant of Jak1 protein inhibits the ability of v-Abl to activate STATs and to induce cytokine-independent proliferation, indicating that an active Jak1 is required for these v-Abl-induced signaling pathways in vivo. It is proposed that Jak1 is a mediator of v-Abl-induced STAT activation and v-Abl induced proliferation in BAF/3 cells, and may be important for efficient transformation of immature B cells by the v-abl oncogene (Danial, 1998).

Human T cell leukemia/lymphotropic virus type I (HTLV-I) induces adult T cell leukemia/lymphoma (ATLL). The mechanism of HTLV-I oncogenesis in T cells remains partly elusive. In vitro, HTLV-I
induces ligand-independent transformation of human CD4(+) T cells, an event that correlates with acquisition of constitutive phosphorylation of Janus kinases (JAK) and signal transducers and activators
of transcription (STAT) proteins. However, it is unclear whether the in vitro model of HTLV-I transformation has relevance to viral leukemogenesis in vivo. In cell extracts of uncultured leukemic cells
from 12 patients with ATLL, the status of both JAK/STAT
phosphorylation and DNA-binding activity of STAT proteins was tested with DNA-binding assays, using DNA oligonucleotides specific for
STAT-1 and STAT-3, STAT-5 and STAT-6 or, more directly, by immunoprecipitation and immunoblotting with anti-phosphotyrosine antibody for JAK and STAT proteins. Leukemic cells from 8 of the 12 patients studied displayed constitutive DNA-binding activity of one or more STAT proteins; the constitutive activation of the JAK/STAT pathway was found to persist over time in the 2 patients followed longitudinally. An association between JAK3 and STAT-1, STAT-3, and STAT-5 activation and cell-cycle progression has been demonstrated by both propidium iodide staining and bromodeoxyuridine incorporation in cells of four of the patients tested. These results imply that JAK/STAT activation is associated with replication of leukemic cells and that therapeutic approaches aimed at JAK/STAT inhibition may be considered to halt neoplastic growth (Takemoto, 1997).

Stromal cell-derived factor-1 (SDF-1), the ligand for the CXCR4 receptor, is a highly efficacious chemoattractant for CD34(+) hematopoietic progenitor cells. However, the SDF-1/CXCR4 signaling pathways that regulate hematopoiesis are still not well defined. This study reports that SDF-1alpha can stimulate the tyrosine phosphorylation of Janus kinase 2 (JAK2) and other members of the
JAK/signal transduction and activation of transcription (STAT) family, including JAK1, tyrosine kinase 2, STAT2, and STAT4 in the human progenitor cell line, CTS. SDF-1alpha stimulation of these cells also enhances the association of JAK2 with phosphatidylinositol 3 (PI3)-kinase. This enhanced association is abolished by pretreatment of cells with AG490, a specific JAK2 inhibitor. Furthermore, pretreatment of CTS cells with AG490 significantly inhibits SDF-1alpha-induced PI3-kinase activity, and inhibition of JAK2 with AG490 ablates the SDF-1alpha-induced tyrosine phosphorylation of multiple focal adhesion proteins (including focal adhesion kinase, related adhesion focal tyrosine kinase, paxillin, CrkII, CrkL, and p130Cas; see CAS/CSE1 segregation protein). Chemotaxis assays show that inhibition of JAK2 diminishes SDF-1alpha-induced migration in both CTS cells and CD34(+) human bone marrow progenitor cells. Hence, these results
suggest that JAK2 is required for CXCR4 receptor-mediated signaling that
regulates cytoskeletal proteins and cell migration through PI3-kinase pathways
in hematopoietic progenitor cells (Zhang, 2001).

Defining signals that can support the self-renewal of multipotential hemopoietic progenitor cells (MHPCs) is pertinent to understanding leukemogenesis and may be relevant to developing stem cell-based therapies. A set of signals, JAK2 plus either c-kit or flt-3, is defined which together can support extensive MHPC self-renewal. Phenotypically and functionally distinct populations of MHPCs are
obtained, depending on which receptor tyrosine kinase, c-kit or flt-3, is activated. Self-renewal is abrogated in the absence of
STAT5a/b, and in the presence of inhibitors targeting either the mitogen-activated protein kinase or phosphatidylinositol 3' kinase
pathways. These findings suggest that a simple two-component signal can drive MHPC self-renewal (Zhao, 2002).

JAK-STAT signalling is required throughout telotrophic oogenesis and short-germ embryogenesis of the beetle Tribolium

In Drosophila, the JAK-STAT signalling pathway regulates a broad array of developmental functions including segmentation and oogenesis. This study analysed the functions of Tribolium JAK-STAT signalling factors and of Suppressor Of Cytokine Signalling (SOCS) orthologues, which are known to function as negative regulators of JAK-STAT signalling, during telotrophic oogenesis and short-germ embryogenesis. The beetle Tribolium features telotrophic ovaries, which differ fundamentally from the polytrophic ovary of Drosophila. While the requirement for JAK-STAT signalling in specifying the interfollicular stalk was found to be principally conserved, it was demonstrated that these genes also have early and presumably telotrophic specific functions. Moreover, the SOCS genes crucially contribute to telotrophic Tribolium oogenesis, as their inactivation by RNAi results in compound follicles. During short-germ embryogenesis, JAK-STAT signalling is required in the maintenance of segment primordia, indicating that this signalling cascade acts in the framework of the segment-polarity network. In addition, it is demonstrated that JAK-STAT signalling crucially contributes to early anterior patterning. It is posited that this signalling cascade is involved in achieving accurate levels of expression of individual pair-rule and gap gene domains in early embryonic patterning (Bäumer, 2011).

Activation of Janus kinase 2 (JAK2) by chromosomal translocations or point mutations is a frequent event in haematological malignancies. JAK2 is a non-receptor tyrosine kinase that regulates several cellular processes by inducing cytoplasmic signalling cascades. This study shows that human JAK2 is present in the nucleus of haematopoietic cells and directly phosphorylates Tyr 41 (Y41) on histone H3. Heterochromatin protein 1alpha (HP1alpha), but not HP1beta, specifically binds to this region of H3 through its chromo-shadow domain. Phosphorylation of H3Y41 by JAK2 prevents this binding. Inhibition of JAK2 activity in human leukaemic cells decreases both the expression of the haematopoietic oncogene lmo2 and the phosphorylation of H3Y41 at its promoter, while simultaneously increasing the binding of HP1alpha at the same site. These results identify a previously unrecognized nuclear role for JAK2 in the phosphorylation of H3Y41 and reveal a direct mechanistic link between two genes, jak2 and lmo2, involved in normal haematopoiesis and leukaemia (Dawson, 2009).

Lymphocyte recruitment is regulated by signaling modules based on the activity of Rho and Rap small guanosine triphosphatases that control integrin activation by chemokines. This study shows that Janus kinase (JAK) protein tyrosine kinases control chemokine-induced Lymphocyte function-associated antigen 1 (LFA-1) and Integrin alpha4beta1 (VLA-4) mediated adhesion as well as human T lymphocyte homing to secondary lymphoid organs. JAK2 and JAK3 isoforms, but not JAK1, mediate CXCL12-induced LFA-1 triggering to a high affinity state. Signal transduction analysis showed that chemokine-induced activation of the Rho module of LFA-1 affinity triggering is dependent on JAK activity, with VAV1 mediating Rho activation by JAKs in a Galphai-independent manner. Furthermore, activation of Rap1A by chemokines is also dependent on JAK2 and JAK3 activity. Importantly, activation of Rap1A by JAKs is mediated by RhoA and PLD1, thus establishing Rap1A as a downstream effector of the Rho module. Thus, JAK tyrosine kinases control integrin activation and dependent lymphocyte trafficking by bridging chemokine receptors to the concurrent and hierarchical activation of the Rho and Rap modules of integrin activation (Montresor, 2013).